Positive reinforcement, just like with cats.
You have to ignore then when they potty in the wrong place, ignore as in not get mad or show any emotion at all.. simply place them back on their perch...
Now, the time consuming part - you must watch them. When you see them potty on the perch you need to shower them with attention and praise. After anywhere from a few days to a few weeks they will come to understand that pottying on the perch is a good thing.. you need to praise them everytime you see them potty on the perch for the first week or two.. then drop every third or fourth praise, slowly weaning down to none, but you must do the weaning randomly - this is how you break their need/desire for praise to them pottying on the perch. Eventually they will want to go to the perch when they want to potty - and now it's up to you to be mindful of them... if they start getting ansty take them to the perch.. they probably want to potty, if they don't go after 30 seconds or so.. continue with what you were doing...
paying attention to them so I understood they needed to potty was probably the hardest part on me... I got bit more than a few times because I wanted to keep playing and they had to go!